#include<iostream>
using namespace std;

int main()
{
	int a1,a2,n,r;
	cin >> a1 >> a2 >> n;
	if(n == 1)
		cout << a1 << endl;
	else if(n == 2)
		cout << a2 << endl;
	else
		r = a1 + (a2 - a1) * (n - 1);
		cout << r << endl;
	return 0;
}
